System for controlling the level of an earth-removing blade of a bulldozer

ABSTRACT

A system for controlling the level of an earth-removing blade of a bulldozer. The system comprises a first circuit means adapted for manual control of the level of the earth-removing blade and a second circuit means automatically operative to keep the earthremoving blade at a predetermined level. The first circuit means is usable in usual earth excavating and removing operation of the bulldozer while the second circuit means is usable to produce a flatly bulldozed earth surface. The second circuit means includes electric and hydraulic circuits. The electric circuit includes an input setting unit, a gyro-means to measure the actual level of the blade, and a circuit for comparing the signals from the input setting unit and gyro-means to emit an instruction signal. The hydraulic circuit includes valves operative in response to the instruction signal to control the amount and direction of the fluid to be fed from a fluid source to a cylinder for the actuation of blade supporting arms of the bulldozer for thereby automatically controlling the blade level.

United States Patent [1 1 Tkeda et al.

[ Aug. 27, 1974 SYSTEM FOR CONTROLLING THE LEVEL OF AN EARTH-REMOVINGBLADE OF A BULLDOZER [75] Inventors: Toshimichi lkeda, lbaraki-Ken;

Masataka Kawauchi; Atsushi Matsuzaki, both of Tokyo; Masayuki Suzuki,Kashiwa, all of Japan [73] Assignee: Hitachi Construction Machinery Co.,Ltd, Tokyo, Japan [22] Filed: Sept. 1, 1972 [21] App]. No.: 285,645

[30] Foreign Application Priority Data 3,674,094 7/1972 Kuntz 172/45Primary Examiner-Stephen C. Pellegrino Attorney, Agent, or Firm-Craig &Antonelli l 5 7 1 ABSTRACT A system for controlling the level of anearthremoving blade of a bulldozer. The system comprises a first circuitmeans adapted for manual control of the level of the earth-removingblade: and a second circuit means automatically operative to keep theearthremoving blade at a predetermined level. The first circuit means isusable in usual earth excavating and removing operation of the bulldozerwhile the second circuit means is usable to produce a flatly bulldozedearth surface. The second circuit means includes electric and hydrauliccircuits. The electric circuit includes an input setting unit, agyro-means to measure the actual level of the blade, and a circuit forcomparing the signals from the input setting unit and gyromeans to emitan instruction signal. The hydraulic circuit includes valves operativein. response to the instruction signal to control the amount anddirection of the fluid to be fed from a fluid source to a cylinder forthe actuation of blade supporting arms of the bulldozer for therebyautomatically controlling the blade level.

3 Claims, 4 Drawing Figures mmmwsz 3.831.583

SIEHBWQ BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention generally relates to a bulldozer and, moreparticularly, to a system for controlling the level of an earth-removingblade pivotally mounted on the forward end of a bulldozer.

2. Description of Prior Art In general, a bulldozer is designed toperform not only a heavy operation in which the bulldozer is operated toscrape raised portions of the ground and forcibly push, transfer or heapup the scraped earth and sand, but also a light operation, such asground conditioning or production of building lots, in which thebulldozer is operated to produce substantially flatly bulldozed orlevelled ground surfaces.

There is an increasing demand for the application of bulldozers to lightoperations, such as ground conditioning, production of building lots, orroad reparing and improvement, in which bulldozers are required toproduce accurately finished substantially flat ground surfaces.

In bulldozer operation to produce a generally flatly levelled groundsurface, it is very difficult and requires skilled technique to manuallykeep the blade of the bulldozer at a controlled substantially fixedlevel because the bulldozer body is subjected to pitching due to ruggedground surface. More specifically, if a ground surface produced byremoval of the earth by means of an earth-removing blade of a bulldozeris once rugged for some reasons, the caterpillars of the bulldozermoving across the rugged ground surface are caused to alternately rideon raised portions of the ground and fall down in recessed portionsthereof for thereby causing pitching of the bulldozer body, with aresult that the ground surface becomes more and more rugged. In orderthat the rugged ground surface may be flatly bulldozed, an operator isrequired to operate an operating lever of the bulldozer to control theearth-removing blade thereof while the operator carefully investigatesthe state of the portion of the ground surface which is locatedforwardly of the earth-removing blade and which is included in the deadangle of view sight as viewed from normal operating position and whilethe operator watches the movement of the earth-removing blade relativeto the rugged ground surface. It takes a great amount of time for a manto become skilled in such complicated control operation. Particularly,the blade control operation is very difficult and needs highly skilledtechnique when the bulldozer moves at a higher speed.

For the reasons, there has been proposed a system for automaticallykeeping the earth-removing blade of a bulldozer at a predeterminedsubstantially fixed level even if the operator of the bulldozer freesthe blade control lever out of control by his hand. For example, US.Pat. No. 3,556,225 issued on Jan. 19, I971 discloses a system forautomatically keeping the earthremoving blade of a bulldozer at apredetermined controlled level. The system utilizes an optical levelreference device disposed outside the bulldozer body, i.e., placed forexample on the ground so that the level reference is followed to controlthe blade level. The automatic blade level control system of this type,however,

fails to provide a satisfactory blade level control and an improvedworking efficiency.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a system for controlling the level of an earth-removing blade ofa bulldozer which enables the blade to pro- It is a further object ofthe present invention to provide a system of the kind specified andwhich is operative to control the bulldozer blade level with goodresponse and controllability.

A still further object of the present invention is to provide a systemof the kind specified and in which a selected one of automatic andmanual controls of the blade level can be performed by the use of aselected hydraulic power source for economical and efficient operationof the system.

In order to achieve the above objects, the present invention provides asystem for use with a bulldozer which has an earth-removing blade, armssupporting said blade and a hydraulically operated means such as acylinder for actuating said blade supporting arms to control the levelof said blade. The system includes an automatic control means comprisingan input setting unit for setting a desired level for the blade andemitting a first signal representing the desired blade level set, ameasuring unit detecting the angle of inclination of the bladesupporting arms relative to a reference plane to measure the actuallevel of the blade and emit a second signal representing the actualblade level measured, and a controlling circuit operative to detect adeviation of the second signal from the first signal and actuate thehydraulic cylinder in such a manner that the deviation is made zero."The input setting unit of the automatic control means is installedwithin a body of the bulldozer rather than being disposed outside thebulldozer body as is the level reference device of the prior art. Theautomatic control means is operative to compare the first and secondsignals from the input setting and measuring units, respectively, andautomatically control the blade substantially at the desired orpredetermined level in such a manner as to compensate for the deviationof the second signal from the first signal. The system of the inventionalso includes a manual controlling means which is manually operable byan operator independently of the automatic control means to control theblade level, i.e., upwardly and downwardly adjust the blade level.

As discussed above, the system of the present inven' tion comprisesindependent controlling means which are operative not only toautomatically control the level of an earth-removing blade of abulldozer substantially at a desired or predetermined level but also tomanually control the blade level. A selected one of the independentcontrolling means can be used for a most suited particular applicationof the bulldozer operation. In other words, the system of the presentinvention enables a bulldozer to be highly efficiently and accuratelycontrolled not only in a usual earth-pushing operation but in anotherkind of operation, such as production of BRIEF DESCRIPTION OF DRAWINGSFIG. 1 is a diagrammatical illustration of the hydraulic circuit of apreferred embodiment of the system according to the present invention;

FIG. 2 is a block diagram mainly showing the electric circuit of thepreferred embodiment of the system according to theinvention;

FIG. 3 illustrates in side elevation the details of an input settingunit employed in the system of the invention; and

FIG. 4 shows a modification of the measuring unit employed in the systemof the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. 1, there isshown a preferred embodiment of the system for controlling the level ofan earth-removing blade of a bulldozer according to the presentinvention. The bulldozer is generally indicated at l and generally shownas having track frames 2 (only one of which is shown), caterpillars 3(only one of which is shown) operatively mounted on the track frames 2,a tractor body 4 mounted on the track frames 2, a blade 5 for removingthe earth, arms 6 (only one of which is shown) pivotally mounted attheir one ends on the track frames 2 for supporting the blade 5 on theother ends, and hydraulic cylinders 8 (only one of which is shown) eachhaving a piston rod 7 secured at one end to a piston in the cylinder 8and pivotally connected at the other end to the associated bladesupporting arm 6 for controlling the level of the earthremoving blade 5.

The illustrated embodiment of the system of the invention includes afirst pump 9 and a second pump 10 having a greater capacity than that ofthe first pump 9. The pumps 9 and 10 form hydraulic pressure sources forthe system of the invention. In a pipe line extending between the firstpump 9 and the cylinder 8 is provided a control valve 11 of continuouscontrol action type. In the illustrated embodiment of the invention, thecontrol valve 11 is preferably a serovovalve. In a pipe line extendingbetween the second pump 10 and the cylinder 8 is provided an on-off typecontrol valve 12, which forms a part of an automatic control means ofthe system of the invention, and a manually operated direction controlvalve 13 forming a part of a manual control means of the system of theinvention. The valves 12 and 13 are in parallel with the servovalve 11.In the illustrated embodiment of the invention, the on-off action typevalve 12 is preferably a solenoid operated valve.

The servovalve 11 and the solenoid valve 12 are operative toautomatically maintain the earth-removing blade 5 at a controlled,substantially predetermined level. This will be called automatic controlhereunder. The direction control valve 13 is manually operable by anoperator to vary the level of the earthremoving blade 5. This will becalled manual control hereunder. The valve 13 is operable by means of anop erating lever which can be held in a locked position by a lockinglever 21 during automatic control of the blade level. The hydrauliccircuit in which the servovalve 11 is provided includes a filter circuit14 provided in the circuit on the outlet side of the servo-valve 11 forfiltering and removing foreign materials such as dust particlescontained in the fluid returned through the circuit from the cylinder 8.The circuit also includes a shut-off valve 16 provided between thefilter circuit 14 and the cylinder 8. The shut-off valve 16 is changedover, when the system of the invention is manually operated, by a returnspring 15 from a position indicated at A in which the servovalve 11 hasbeen communicated through the shut-off valve 16 with the cylinder 8 to aposition indicated at B in which the communication between theservovalve 11 and the cylinder 8 is blocked by the shut-off valve 16. Onthe outlet side of the first pump 9 in the hydraulic circuit includingthe servovalve 1 1 is provided an unloading circuit 19 which isoperative to unload and return the hydraulic fluid from the pump 9 to areservoir 20 during manual control of the system of the invention. Theunloading circuit 19 includes a pressure control valve 17 and a solenoidoperated valve 18 which serves as a pilot valve for the pressure controlvalve 17. As shown in FIG. 1, the filter circuit 14 comprises acombination of conventional check valves 14a and filters 1412.

When the afore-described hydraulic circuit of the system of the presentinvention is operated to level the surface of the earth, i.e., when thehydraulic circuit is operated to automatically control the level of theearthremoving blade 5 so that the cutting edge of the blade 5 ispositioned in a reference plane which, for example, may be the plane inwhich the treads of the caterpillars of the bulldozer are included, theservovalve 11 and the solenoid valve 12 are automatically operative tomake zero the deviation of a value representing the actual level of theblade 5 relative to another value representing a predetermined level ofthe blade 5 with respect to the reference plane.

The above-described automatic control can be performed by an electriccircuit shown in FIG. 2 in which reference numerals the same as thoseused in FIG. 1 represent the same parts as those shown in FIG. 1. Theelectric circuit for the automatic control includes an input settingunit 22 for setting a desired level of the blade 5 and emitting anelectrical signal representing the desired blade level set, a measuringunit 23 operative to detect the actual angle of inclination of the bladesupporting arms 6 with respect to the horizontal and measure the actuallevel of the blade 5 on the basis of a value representing the actualinclination of the arms for emitting a second signal representing theactual blade level measured, a comparator or comparing circuit 24operative to compare the signal from the input setting unit 22 with thesignal from the measuring unit 23 for determining the deviation of thelatter signal from the former one and for emitting operatinginstructions to the servovalve 11 or to the solenoid valve 12 inaccordance with the amplitude of the deviation, an electric power source25, a switch 26 for the electric power source, means 27 for detecting atrouble in the automatic control circuit used in the course of anautomatic control of the system of the invention and means 28 forchanging the hydraulic circuits when an automatic control is changed toa manual control or vice versa.

In the embodiment of the invention shown in FIGS.

1 and 2, the measuring unit 23 is preferably mounted on a bladesupporting arm 6 to measure the angle of inclination of the arm 6 withrespect to a reference plane so that the level of the blade 5 isdirectly measured on the basis of the angle of inclination of the arm 6.

A description will be made with respect to the details of the elementsof the above-described electric circuit. As will be seen in FIG. 3, theinput setting unit 22 comprises a potentiometer 29 which is adisplacementvoltage transducer, a lever 31 operative to move a contact30 of the potentiometer 29, a friction disc 33 mounted on a pivot 32 ofthe lever 31, a breaking member urged by a spring 34 against thefriction disc 33. The input setting unit 22 is arranged such that theangular movement of the lever 31 for an angle about an axis of the pivot32 is operative to rotate the contact 30 for causing the potentiometer29 to emit an electric signal corresponding to the angle of desiredinclination of the blade supporting arms 6. When the angular move mentof the lever 31 is discontinued at an angular position, the lever 31will be held in that position by the cooperation ofthe friction disc 33and the breaking member 35.

The measuring unit 23 is mounted on a blade supporting arm 6 in such amanner that the unit 23 is guarded against exterior shock and foreignmaterial suchas dirt and sand particles. The measuring unit 23preferably include several attachments such as a detector 36 comprisinga gyroscope, means 37 for driving the gyroscope, means 38 for unlockingthe gyroscope in stationary position, and means 39 for holding orlocking the gyroscope. The measuring unit 23 is arranged such that, whenthe switch 26 is switched on, the gyro unlocking means 38 is operatedand the detector 36 is driven by the gyro drive motor 37 to measure theangle of inclination of the arms 6 and, when the switch 26 is switchedoff, the operation of the motor 37 is discontinued and the gyro lockingmeans 39 is operated to lock the gyroscope. The measuring unit 23 may beformed of gyro units of the type disclosed in Japanese Pat. PublicationNo. 3699/1962 published on June 6, 1962 and relating to A Method ofStarting Operation of a Gyrocompass," Japanese Utility Model PublicationNo. 22601/1970 published on Sept. 7, 1970 and relating to A GyroClamping Mechanism, and Japanese Pat. Publication No. 298/1962 publishedon Jan. 25, 1962 and relating to An Apparatus for Centering and Holdinga Gyroscope, while the detector 36 may be of the type disclosed inElectric Measurement Handbook" by Jim Yamauchi, 3rd print published onMay 30, 1970, paragraph 2.2.1 Orientation and Position MeasuringDevices, pages 984 to 987. Altematively, the measuring unit 23 maycomprise a pendulum or a manometer.

The comparing circuit 24 includes an adder 40 which detects thedeviation of the signal from the gyro-means 23 with respect to thesignal from the input setting unit 22 and emits a signal either toinstruct the servovalve 11 to operate when the deviation is lesser thana predetermined value, or to instruct both servovalve 11 and solenoidvalve 12 to operate when the deviation is greater than the predeterminedvalue..The comparing circuit 24 also includes a servo amplifier 41 ofthe type disclosed in US. Pat. No. 3,000,121 issued Sept. 19, 1961 whichis operative to amplify the servovalve operation instruction signal fromthe adder 40 and send the amplified signal to the servovalve 11, and anamplifier 42 of the type disclosed in US. Pat. No. 3,556,225 issued Jan.19, 1971 which is operative to amplify the solenoid valve operationinstruction signal from the adder 40 and supply the amplified signal tovalve 12.

The trouble detecting means 27 includes a block detector 43 operative inresponse to an accidental block of the filter circuit 14 on the outletside of the servovalve 11 to switch-off the electric power source switch26.

The block detector 43 detects blockage of the filter circuit 14 on thebasis of the pressure differential between the inlet and outlet sidesthereof and operates a relay to control switch 26 and may be of the typedisclosed in Japanese Utility Model Publication No. 32050/1968 publishedon Dec. 26, 1968 and relating to A Device for Indicating Block in aFilter. The detector means 27 also includes a gyro nonoperation detector44 operative in response to the inclination of the blade support arms 6beyond the measuring capacity of the gyroscope to switch off the switch26 and, at the same time, cause the gyro locking means 39 to operate tolock the gyroscope.

The hydraulic circuit changing means 28 includes a shut-off valveactuator 45 which is operative, during automatic ocntrol of theearth-removing blade 6, to retain the shut-off valve 16 in a position Aagainst the return spring 15 so that the servo valve 11 is incommunication with the blade operation cylinder 8. The changing means 28also includes an unloading circuit actuator 46 by which the unloadingcircuit 19 is maintained in a position in which the hydraulic fluid fromthe first pump 9 is not unloaded. The actuators 45 and 46 may beamplifier arrangements which provide an amplified signal necessary forthe control of the valve 16 and circuit 19 and may be of the typedesclosed in An Electric-Hydraulic System for Controlling the Turning-upDepth of a Plow Mounted on a Tractor which is an introduction byJapanese Magazine Hydraulics and Pneumatics, Vol. 8, No. 3, published inMarch, 1970. The detector 36 of the gyro-means 23 detects the voltage ofelectric current from a voltage regulator 47 and apply the detectedvoltage to the adder 40 of the comparing circuit 24. The actuators 45and 46 and the voltage regulator 47 are electrically connected through aswitch 49 and a delay switch 50 to the power source switch 26. Theswitch 49 is connected to an automaticmanual selector 48 provided forthe operating lever 31 of the input setting unit 22. The switch 49 hascontacts which are opened when the automatic-manual selector 48 ischanged-over to a manual control position to thereby open the circuitfor the supply of an operation signal to the actuators 45 and 46 and thevoltage regulator 47. In response to the opening of the contacts of theswitch 49, the shut-off valve 16 is moved by the return spring 15 to itsB position in which the communication between the servovalve 11 and thecylinder 8 is shut-off while the unloading circuit 19 discharges thehydraulic fluid from the first pump 9 into the reservoir 20 to minimizethe power loss of the first pump 9.

In the course of the automatic control of the level of theearth-removing blade 5, since: the hydraulic pipe line for the controlof the blade level is so arranged as to appropriately adjust the bladeto a desired level by the manual operation of the direction controlvalve 13, the closing of the power source switch 26 energizes thegyro-drive motor 37 of the gyro-means 23 so that the gyro-detector 36 isdriven by the gyro-drive motor 37. When the gyro-detector 36 attains astate sufficiently stable to detect the angle of inclination of theblade supporting arms 6, the contacts of the delay switch 50 are closed.When the automatic-manual selector 48 is set in the automatic controlposition and the switch 49 is actuated to close its contacts, theelectric current from the power source 25 is fed to the actuators 45 and46 and the voltage regulator 47. The actuator '45 will then emit aninstruction signal whereby the shutoff valve 16 is changed-over from theB position to the A position in which the servovalve 11 is incommunication with the cylinder 8 for the operation of the blade 5.Similarly, the actuator 46 will emit an instruction signal by which theunloading circuit 19 is made inoperative to unload the hydraulic fluidfrom the first pump 9 into the reservoir 20, whereby the fluid is fedfrom the pump 9 to the servovalve 11 so that the system attains a statein which it is prepared for the automatic control of the level of theearth-removing blade of the bulldozer. In this state of the system, thebulldozer may be laid on a substantially horizontal surface of theground. Assuming that the substantially horizontal surface mentionedabove is selected as the reference plane, the blade 5 is so adjustedthat the cutting edge of the blade is positioned substantially at thesame level as the treads of the caterpillars 3. If the input settingunit 22 is so set that the deviation of the signal from the gyro-means23 relative to the signal from the input setting unit 22 is zero," theforward movement of the bulldozer causes the blade 5 to produce an earthsurface which is substantially co-plannar or flash with the referenceplane. In a case where the level of the blade 5 is varied with respectto the reference plane during forward movement of the bulldozer due, forexample, to rugged ground surface, the variation is detected by thecomparing circuit 24 in the form of a deviation of the signal from thegyro-means 23 with respect to the signal from the input setting unit 22.The adder 40 of the comparing circuit 24 operates toemit and feedthrough the servo amplifier 41 to the servovalve 11 an instructionsignal for causing the servovalve 11 to operate to compensate for thedeviation, i.e., to make the deviation zero, with a result that theservovalve 11 is moved from its neutral position to one of its operativepositions so that the hydraulic fluid from the first pump 9 isintroduced into the cylinder 8 through one of the inlets thereof untilthe level of the cutting edge of the blade 5 is adjusted to be the sameas the reference plane. When the blade 5 has attained this position, theadder 40 discontinues emitting the change-over instruction signal to theservovalve 11 so that the latter is restored to its neutral position. Asis apparent to those in the art, the servovalve 11 has an inherentcharacteristic that, even if it is in its neutral position, theservovalve 11 is inoperative to block or shutoff the hydraulic fluidbeing supplied to the blade operating cylinder 8. Thus, the cylinder isoperable by the hydraulic fluid from the first pump 9 to keep the bladein the adjusted level.

The automatic control above described may be performed in a case wherethe variation of the level of the earth-removing blade 5 relative to thereference plane is lesser than a predetermined value. In a case wherethe variation is greater than the predetermined value, the adder 40 ofthe comparingcircuit 24 will emit and feed through the servoamplifier 41and the solenoid enaccordance with the direction in which the variationof the blade level has taken place, to thereby adjust the varied levelof the blade 5. The greater amount of the hydraulic fluid from thesecond pump 10 in combination with the smaller amount of that from thefirst pump 9 are effective to quickly compensate for the greatervariation of the level of the earth-removing blade 5 from the referenceplane. The system is operative to repeat the described controllingoperation to enable 'the blade 5 to produce a substantially levelled orflatly bulldozed earth surface.

If the filter circuit 14 is blocked in the course of an automaticcontrol of the blade level, the block detector 43 detects the block ofthe circuit 14 and operates to open the contacts of the power sourceswitch 26. In addition, if the angle of inclination of the bladesupporting arms 6 relative to the reference plane exceeds the rangewithin which the gyro-means 23 is operative to measure the inclination,the gyro-non-operation detector 44 detects the non-operation of thegyroscope and operates to open the contacts of the power source switch26 to interrupt the automatic control of the blade level, by which theoperator can be advised of the trouble in the circuit for the automaticcontrol.

The control of the level of the earth-removing blade 5 by means of theafore-described automatic control circuit can be performed in thefollowing manner: The operating lever 31 for the input setting unit 22may be operated to move the contact of the potentiometer 29 of the unit22 to a position which corresponds to a desired set value on the inputsetting unit 22. The servovalve 11 and the solenoid valve 12 areoperative to control the amount and direction of the hydraulic fluid tobe fed into the cylinder 8 in such a manner that the deviation of theangle of inclination of the blade supporting arms 6, as measured by thegyro-means 23, relative to the set value is made substantially zero,with a result that the blade 5 is moved to a level corresponding to thevalue set on the input setting unit 22. In such manner, any desiredvalue may be set in the input setting unit 22 for a desired angle ofinclination of the arms 6 relative to the reference plane to enable thebulldozer to be operative to excavate and remove the earth as aconventional bulldozer is.

Selective operation of the automatic-manual selector 48 makes itpossible to obtain repeated and alternate performance of automatic andmanual controls of the earth-removing blade 5. More specifically, if theautomatic-manual selector 48 is moved to its manual control position,since the contacts of the switch 49 are open and the contacts of theswitch 26 are closed, the gyro means 23 is driven by the gyro drivemotor 37 and is maintained in its measuring position. However, as theoperation instructing signal to the voltage regulator 47 is shutoff, thegyro-means 23 does not feed to the adder 40 of the comparing circuit 24a signal obtained from the detection of the angle of inclination of theblade supporting arms 6. Since the operation instructing signal to theshut-off valve actuator is shut off,

the shutoff valve 16 is moved by the return spring to the B position toshut-off the communication between the servovalve Ill and the cylinder8. As the operation instructing signal to the unloading circuit actuator46 is shut off, the unloading circuit 19 is in a position in which thefluid from the first pump 9 is not passed to the servovalve 11 butreturned to the reservoir 20. Thus, the circuit for the automaticcontrol of the blade level is inoperative. On the other hand, themanually operated direction control valve 13 is operative to actuate thecylinder 8. The operating lever for the direction control valve 13 maybe unlocked to move the valve 13 to a desired position so that theoperatorcan manually control the level of the earthremoving blade 5. Ifthe automatic-manual selector 48 is then moved to an automatic controlposition, the contacts of the switch 49 are closed to allow the electriccurrent from the power source to be fed to the actuators 45 and 46 andthe voltage regulator 47 so that the gyro means 23 is maintained in itsmeasuring position and the shut-off valve 16 is moved to its A positionin which the servo valve 11 is communicated with the blade operationcylinder 8 and the unloading circuit 19 does not unload the hydraulicfluid from the first pump 9 to the reservoir 20 but is operative to feedthe fluid to'the servo valve 11 for the automatic operation of thesystem of the invention.

The manually operated direction control valve 13 alone may be used tomanually control 'the level of the blade 5. In this case, the contactsof the power source switch 26 are opened to make inoperative thegyromeans 23, the comparing circuit 24, the servovalve 11 and thesolenoid valve 12. The gyro-locking means 39 are operative in responseto the opening of the power source switch 26 to lock or hold thegyro-means 23.

As described above, the system of the present invention has a bladelevel control circuit which is automatically operative, regardless ofthe inclination of the bulldozer body substantially in a vertical planeincluding the longitudinal axis of the body, to control the angle ofinclination of the blade supporting arms relative to a reference planefor thereby maintaining the blade substantially at a predetermined,desired level. The automatic control circuit has incorporated therein aninput setting unit by means of which a set value may be appropriatelyvaried to lift or lower the blade in a manner substantially similar tothat in which the blade of a conventional bulldozer is operated.Moreover, the system of the invention includes a manually operateddirection control valve which is manually operable to control the bladeas in conventional bulldozer. Thus, it will be appreciated that thebulldozer which employs the system of the invention can have a widenedrange of application and operation.

Furthermore, the system of the invention can make use of selected one ofhydraulic power souurces in the course of automatic or manual control ofthe blade level. This not only minimizes the power loss but alsoprovides an improved controllability of the earthremoving blade of abulldozer.

The system of the invention may be modified within the spirit of theinvention. FIG. 4 shows a modification of the invention which comprisesan inclination detector 51 mounted on the body 4 of the bulldozer l todetect the inclination of the bulldozer body 4 relative to a referenceplane and a position detector 52 mounted on a blade supporting am 6 todetect the inclination of the arm 6 relative to the bulldozer body sothat the level of the earth-removing blade 5 is indirectly mea sured onthe basis of the signals from the detectors 51 and 52.

In the illustrated embodiment of the invention, the input setting unitis of proportional action type while the control valves in the controlcircuits are of on-off type and continuous control action type,respectively. However, the input setting unit may alternatively be ofintegral control type and the control valves both may alternatively beof either on-off action type or continuous control type.

What is claimed is:

l. A system for controlling the level of an earthremoving blade of abulldozer, comprising:

a. a hydraulic fluid source for feeding a hydraulic fluid under pressureto a cylinder operative to up wardly and downwardly actuate armssupporting said blade,

b. a manual controlling means including a manually operated directioncontrol valve located between said hydraulic source and said cylinder tochange over the direction of feed of said hydraulic fluid from saidhydraulic source to said cylinder for controlling the level of saidblade, and

c. an automatic controlling means including a controlling circuit whichcomprises means for setting a desired level for said blade including aninput setting unit emitting a first signal representing the de* siredblade level set, a measuring unit on said bulldozer detecting the angleof inclination of said blade supporting arms to measure the actual levelof said blade and emitting a second signal representing the measuredactual blade level, a comparing circuit responsive to said first andsecond signals and operative to detect a deviation of said second signalfrom said first signal and emitting a third signal representing thedeviation, on-off action type and continuous control action type controlvalve means located between said hydraulic source and said cylinder andoperative in response to said third signal to control the amount anddirection of said hydraulic fluid to be fed into said cylinder in such amanner as to compensate for said deviation to thereby automaticallycontrol the level of said blade.

2. A system as claimed in claim 1, in which said hydraulic fluid sourcecomprises:

a. A first pump located between said hydraulic source and saidcontinuous action type control valve means and in fluid communicationtherewith for feeding an amount of said hydraulic fluid to saidcontinuous control action type control valve means, and

b. a second pump located between said hydraulic source and said on-offaction type valve means and direction control valve and in fluidcommunication therewith for feeding a larger amount of said hydraulicfluid than that by said first pump to said onoff action type valve meansand said manually operated direction control valve.

3. A system as claimed in claim 1, in which said measuring unit on saidbulldozer included in said automatic controlling means comprises adetector mounted on a blade supporting arm of said bulldozer to detectthe angle of inclination of said arm for measuring the level of saidblade.

1. A system for controlling the level of an earth-removing blade of abulldozer, comprising: a. a hydraulic fluid source for feeding ahydraulic fluid under pressure to a cylinder operative to upwardly anddownwardly actuate arms supporting said blade, b. a manual controllingmeans including a manually operated direction control valve locatedbetween said hydraulic source and said cylinder to change over thedirection of feed of said hydraulic fluid from said hydraulic source tosaid cylinder for controlling the level of said blade, and c. anautomatic controlling means including a controlling circuit whichcomprises means for setting a desired level for said blade including aninput setting unit emitting a first signal representing the desiredblade level set, a measuring unit on said bulldozer detecting the angleof inclination of said blade supporting arms to measure the actual levelof said blade and emitting a second signal representing the measuredactual blade level, a comparing circuit responsive to said first andsecond signals and operative to detect a deviation of said second signalfrom said first signal and emitting a third signal representing thedeviation, on-off action type and continuous control action type controlvalve means located between said hydraulic source and said cylinder andoperative in response to said third signal to control the amount anddirection of said hydraulic fluid to be fed into said cylinder in such amanner as to compensate for said deviation to thereby automaticallycontrol the level of said blade.
 2. A system as claimed in claim 1, inwhich said hydraulic fluid source comprises: a. A first pump locatedbetween said hydraulic source and said continuous action type controlvalve means and in fluid communication therewith for feeding an amountof said hydraulic fluid to said continuous control action type controlvalve means, and b. a second pump loCated between said hydraulic sourceand said on-off action type valve means and direction control valve andin fluid communication therewith for feeding a larger amount of saidhydraulic fluid than that by said first pump to said on-off action typevalve means and said manually operated direction control valve.
 3. Asystem as claimed in claim 1, in which said measuring unit on saidbulldozer included in said automatic controlling means comprises adetector mounted on a blade supporting arm of said bulldozer to detectthe angle of inclination of said arm for measuring the level of saidblade.